The present invention relates to laser drilling in general, and particularly to laser drilling holes in refractory materials, such as alumina and ceramic. A focused or coherent energy source such as a laser beam is often used to cut precision patterns or drill holes in refractory materials. The process of cutting these types of materials is not one of mechanically abrading material, but heating the material with the laser beam and vaporizing it. The vaporized material is spewed out of the hole or kerf, and sometimes redeposits on the surrounding cooler surfaces.
The process of vaporizing is not a uniform one, resulting in holes that are wide at the top and taper down to a narrower size at a distance into the material. This is shown in FIGS. 1 and 2 which illustrate a substrate 10 having laser drilled holes 11. The larger diameter openings 12 appear at the top of the substrate 10. The diameter tapers to the more uniform diameter of the holes 11 at a distance from the top surface (i.e. the side to which the laser beam was applied). For example, for a ceramic substrate 10 having a thickness of 0.025 inches it may be desired to provide holes 11 having a diameter of 0.005 inches, spaced 0.010 inches center to center. The openings 12 can be from 0.003 to 0.005 inches greater than the desired hole diameter. This area of greater diameter can extend for a depth of 0.005 to 0.008 inches before tapering to the desired diameter.
When drilling holes that are closely spaced, this taper effect is particularly, undesirable, in that the edges of the enlarged holes can touch or overlap. Cutting precision patterns is also hampered by this phenomenon, severely restricting the potential use of lasers in creating precision patterns.